1. Field of the Invention
The present invention relates to a road friction coefficients estimating apparatus for a vehicle for estimating friction coefficients on road surfaces and more particularly to a road friction coefficients estimating apparatus for accurately estimating friction coefficients on road surfaces over a wide range of vehicle motion.
2. Discussion of Prior Arts
In recent years, numerous vehicle control technologies such as a traction control technology, a braking force control technology, a torque distribution control technology and the like, have been proposed and some of these control technologies have been realized in actual automobile markets. Many of these control technologies use friction coefficients on a road surface (hereinafter, referred to as xe2x80x9croad friction coefficientxe2x80x9d) for calculation or correction of control parameters. Accordingly, in order to execute the control properly, it is necessary to estimate an accurate road friction coefficient.
There are several technologies in which road friction coefficients are estimated. According to some technologies, road friction coefficients are estimated based on the comparison between an actual vehicle motion obtained from sensor inputs and a vehicle behavior obtained from a vehicle motion model. For example, the applicant of the present invention proposes a technology in which road friction coefficients are estimated by utilizing the adaptive control in Japanese Patent Application Laid-open No. Toku-Kai-Hei 8-2274. Also, the applicant of the present invention proposes a technology in which road friction coefficients are estimated based on the comparison of a slip angle of a vehicle estimated from an observer with a reference value on an road surface with high friction coefficient or an road surface with low friction coefficient respectively in Japanese Patent Application Toku-Gan-Hei 10-242030.
The technology in which road friction coefficients are estimated using a vehicle motion model is formed on the premise that the actual vehicle motion obtained from sensor inputs agrees with the behavior of the vehicle motion model under a specified condition (friction coefficient between tire and road surface is the same).
However, it is difficult to make the actual vehicle motion and the behavior of the vehicle motion model completely agree because of noises contained in the sensor inputs and errors in numerically modeling the vehicle. That is, these noises and errors produce the difference from an actual road friction coefficient.
Accordingly, in case of the technology in which road friction coefficients are estimated using vehicle motion models, it is desirable that road friction coefficients are estimated when the difference of road friction coefficients is substantially noticeable in the vehicle motion, compared to the effects of noises of sensor inputs and errors in modeling vehicle motions. Hence, it is difficult to estimate road friction coefficients accurately over a wide range of vehicle motion.
It is an object of the present invention to provide a road friction coefficients estimating apparatus capable of accurately estimating road friction coefficients over a wide range of vehicle motion.
In order to achieve the object, the road friction coefficients estimating apparatus for a vehicle comprises a first road friction coefficient estimating means for calculating a first road friction coefficient based on a vehicle behavior in a lateral direction, a second road friction coefficient estimating means for calculating a second road friction coefficient based on a vehicle behavior in a longitudinal direction, a third road friction coefficient estimating means for calculating a third road friction coefficient based on road conditions and an estimating value selection means for selecting either one of the first, second and third road friction coefficients as a final road friction coefficient estimating value.